Injection molding process control

ABSTRACT

Both a method and apparatus are disclosed for controlling a cyclically operated injection molding machine such that uniform, high quality molded articles are repeatedly produced from plasticized synthetic resinous material. The method and apparatus include integration of a sensed injection pressure with respect to time from the beginning of an injection stroke until plasticized material in a mold reaches a predetermined pressure. The integration defines a work index which is compared with a preset allowable range. If the work index lies outside the preset range, an adjustment is made to the back pressure acting on the plasticizing screw during the plasticizing portion of the subsequent molding cycle. The apparatus is permitted to make a predetermined number of adjustments in the back pressure after which a suitable indication is made that the molding apparatus is out of control itself. Other viscosity-affecting parameters may be controlled including injection screw rotary speed and injection screw velocity, and barrel temperature of the plasticizing apparatus. A mold cavity pressure sensor indicates that the cavity is full, and thereupon stops the integration and may also reduce the injection pressure to a lower value, sufficient to supply additional material to compensate for shrinking due to cooling, yet not high enough to introduce strains, or cause flashing.

This is a division of application Ser. No. 516,501 filed Dec. 21, 1974.

BACKGROUND OF THE INVENTION

The invention is generally concerned with the automatic control ofinjection molding machines. More specifically, this invention relates toa control for an injection molding machine in which a work-indicativeparameter is integrated with respect to time during the injection ofplasticized material into a mold thus defining a work index which isused to control the molding machine.

In the past it has been common to employ monitoring systems and controlsin injection molding apparatus. Typically, a monitor comprises apparatusfor recording specified variables during each cycle of the injectionmolding apparatus. Such monitoring systems, however, require manualchanges to be made by an operator to obtain consistently uniformproducts. Ordinarily, the operator reviews the recorded variables forconsecutive molding cycles and then makes an appropriate adjustment tothe injection molding apparatus such that acceptable products are likelyto be obtained.

Control systems comprise another type of automatic control for injectionmolding machines. Typically, control systems sense one or moreparameters which are considered to be necessarily associated with theinjection molding of uniform, high quality products. Each of the sensedparameters is compared with a specified range within which the parametermust lie in order to give acceptable products. If a sensed parameterlies outside of its allowable range, a feedback system is ordinarilyprovided to make a compensating adjustment to an operating parameter ofthe injection molding machine.

With monitoring systems and their reliance on human operators, there isa substantial likelihood that a large number of non-uniform, or lowquality products may be molded before the injection molding apparatuscan be brought within acceptable limits. It is noted in passing thatunacceptable products generate excessive manufacturing costs and wastecostly plastic materials.

Control systems, by comparison, substantially reduce the number ofunacceptable products by automatically making compensatory adjustmentswhen necessary. As noted above, control systems typically senseparticular parameters and maintain those parameters within specifiedlimits. While some such controls are useful, there has persisted a needfor a truly practical one capable of maintaining product quality even inthe face of variations in feed stock and ambient conditions. Feed stockof synthetic resinous material varies from one batch to another andfrequently contains varying proportions of virgin and reground material.Both of those characteristics may adversely affect product quality.Ambient temperature is still another variable which can affect theoperation of injection molding apparatus and product quality. The aboveare but a few of the variables which can affect the molded parts and theminimum range which may be specified for the sensed parameters.

Aside from the variables which affect the minimum range for the sensedparameters, there remains the problem of which parameter or parametersare to be sensed as being the most representative of a quality product.A myriad of such parameters have been proposed for use heretofore,including many of the operating parameters of the injection moldingapparatus such as injection ram velocity, melt temperature, meltpressure, hydraulic injection pressure, injection time interval, moldpressure, etc.

Some molding process controls have employed sensing devices to indicatewhen a mold cavity is filled to a specified pressure. Such sensingdevices have been used to shift molding apparatus from an injectionportion to a holding portion of the molding cycle. In addition suchsensing devices have been used as an indication of product quality.

It has also been proposed to indicate product quality by an integral ofwork-parameter integrated with respect to time between positions at thebeginning and the end of a ram stroke. Typically, the selected positionshave been indicated by using a linear position potentiometer. Thisintegral has been fraught with difficulty, however, since the end orbottom of a ram stroke is variable from cycle to cycle. Accordingly, aphysically fixed position for the end of the stroke may allowsignificant product quality variations between consecutive mold cycles.

The use of mold pressure sensors alone has, likewise, been problematic.More specifically, there may be a sink region in the mold or anexcessively packed region. In addition, other parameters of the moldingapparatus may manifest quality-affecting excursions which are notreflected in the selection of mold pressure.

It is therefore apparent that these previously used parameters have notbeen altogether satisfactory for controlling injection moldingapparatus.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novelinjection molding process control which derives an index correlatablewith product quality over a range of variations of conditions and usesthis index to regulate the operation of an injection molding machine.

In accordance with a preferred embodiment of the present invention,hydraulic injection pressure, which translates a plasticizing screw toinject a quantity of molten material into a mold, is integrated withrespect to time during the injection portion of each molding cycle anduntil a predetermined pressure is attained by molten materialaccumulating in the mold cavity. This integration yields a work indexwhich is indicative of the work required to inject the molten materialthroughout the injection portion of the cycle and thereby represents thenet effect of all viscosity perturbations during injection. In additionthe integration automatically responds to variations of materialproperties, to machine or mold variations and to ambient conditions, allof which may affect sequential molding cycles. The integration limitsused here also obviate the effect of vagaries in the physical length ofeach injection stroke and the precise physical position where theinjection stroke ends by eliminating reliance thereon.

The integration preferably ends simultaneously with the occurrence of aselected mold cavity pressure such that the combination of the workindex and the selected mold cavity pressure assure the repetitivemolding of consistently uniform products. By placing a mold cavitysensor at an extremity of the cavity, the sensor is also effective toindicate that the mold has been filled which is also a requirement forconsistent product quality.

Having obtained an index of the work expended, the index is comparedwith an adjustable pre-set allowable range therefor. This comparisonpermits a continuous evaluation of injection apparatus performanceduring consecutive molding cycles while also making possible a record ofmolding machine operation. In the event that the index does not fallwithin the allowable range, the comparison may be used to generate asignal to adjust an appropriate parameter of the molding machine.

When the index for a particular cycle does not lie within the allowablerange, a compensating adjustment is made to a viscosity-affectingparameter such as hydraulic back pressure exerted on the plasticizingscrew during the plasticizing portions of a cycle. The adjustment ismade for subsequent cycles and thus compensates for all viscosityperturbations of the previous cycle. Moreover, this adjustment procedureaccommodates gradual changes in slowly changing variables such asambient temperature in addition to relatively rapid fluctuations whichmay result from changes in material composition.

Another feature that may be incorporated into the process control of thepresent invention provides a predetermined number of molding cycleswithin which the index is permitted to seek a value within the allowablerange. If the index does not adjust itself within the number of cycles,an appropriate indication may be made to signal that the machine is outof control. This feature may be particularly advantageous inapplications where a single operator is charged with simultaneouslymonitoring several molding machines.

With some work-affecting parameters, it may be advantageous to delaycorrective changes so that spurious changes are not made in response tooccasional erratic calculations of the work index. Accordingly, thepresent invention may also be provided with a delay circuit whichpostpones any change until a selected number of consecutive cyclesrequire change.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be apparent to thoseskilled in the art when the appended claims are read in light of thefollowing description and the drawings in which:

FIG. 1 is a diagrammatic view, in partial cross section, of an injectionmolding machine connected to a process control according to the presentinvention; and

FIG. 2 is a diagrammatic illustration of the control system.

DETAILED DESCRIPTION OF THE INVENTION

Depicted in FIG. 1 is a cyclically operated injection molding apparatuscomprising plasticizing apparatus 10, a suitable mold and a processcontrol 50. The plasticizing apparatus 10 includes a hopper 12 whichreceives particulate synthetic resinous material for delivery to areciprocable rotary screw 16 that is mounted internally of a barrel 18wherein the material is plasticized and pressurized. Wrapped around theexternal circumference of the barrel 18 are a plurality of band heatingelements 20, 22, 24. At the outlet end of the barrel 18, a nozzlesection is provided which includes an external band heating element 26.

The nozzle section of the barrel 18 communicates with a mold comprisinga stationary mold portion 28 and a movable mold portion 30. Thestationary mold portion 28 and the movable mold portion 30 cooperate todefine a mold cavity therebetween into which plasticized material isinjected from the plasticizing apparatus 10 such that a molded articleor product results.

At the end of the screw 16 opposite from the mold, suitable apparatus isprovided to both rotate and reciprocate the screw 16 relative to thebarrel 18. More specifically, a screw drive motor 32 is provided toimpart rotary motion to the screw 16 during a plasticization portion ofa molding cycle. In addition, the screw 16 is provided with a piston end34 which is mounted internally of a cylinder member 36 such that achamber 38 is defined therebetween. The chamber 38 is supplied withpressurized hydraulic fluid from a reservoir 41 through a suitable pump40.

The pump 40 provides a high pressure high volume flow of hydraulic fluidto the chamber 38 to reciprocate the screw 16 during an injectionportion of the molding cycle, and supplies a low volume low pressureflow of hydraulic fluid to the chamber 38 during a holding portion ofthe molding cycle. Communicating with the chamber 38 and a reservoir 41is a suitable back pressure control 42 which provides a variableadjustment of the back pressure acting in the chamber 38. The backpressure control 42 may comprise a conventional electric relief valve.

To control the flow of hydraulic fluid from the pump 40 to the chamber38, a control valve 46 is preferably provided. The valve 46 may comprisea conventional electric control valve and is operable to control boththe volume and pressure of hydraulic fluid in chamber 38.

As illustrated in FIG. 1, a second pump 43 may be provided to supplyhydraulic fluid to the hydraulic motor 32 which rotates the screw 16. Asuitable screw speed control 44 is provided between the pump 43 and thehydraulic motor 32 for regulating the speed thereof. The screw speedcontrol 44 may be a conventional electric flow control valve.

The mold cavity between mold portions 28, 30 is provided with a suitablesensing means 48 which is preferably disposed at an extremity of themold cavity remote from the plasticizing apparatus 10. The mold cavitypressure sensor 48 measures the pressure of plasticized material whichhas been injected into the mold. Moreover, by positioning the sensor 48at an extremity, the sensor 48 ensures that the sensed pressurerepresents the pressure in a filled mold cavity. The pressure sensingmeans 48 is operable to generate a signal representing plasticizedmaterial pressure and to communicate this material pressure signal tothe process control apparatus 50.

The chamber 38 includes an injection pressure sensing means 52 whichgenerates a signal that is indicative of the hydraulic pressure in thechamber 38. This injection pressure signal from the pressure sensingmeans 52 is also input into the process control apparatus 50. Thereciprocating screw 16 may also be provided with anintegration-initiating position transducer 54 which indicates theposition of the screw 16 with respect to the barrel 18. The linearposition transducer 54 provides an input signal for the process controlapparatus 50.

Turning now to FIG. 2, the process control apparatus 50 includes asettable selected mold cavity pressure input 60 that establishes apredetermined pressure which is to be attained by plasticized materialwithin the mold cavity. A predetermined allowable range for a work indexis input to the process control apparatus 50 by setting a high limit 62and a low limit 63.

An out-of-control input 64 enters a predetermined number of consecutivemolding cycles within which the control apparatus 50 is expected tobring the work index calculated for each cycle within the rangespecified by the limits 62, 63. In the event that the control apparatus50 fails to bring the work index within the specified allowable rangewithin the predetermined number of cycles, a suitable alarm means 108may be provided to summon assistance from an operator.

An increment size adjustment 66 specifies the magnitude of anincremental change which is to be made in a viscosity-affectingparameter for subsequent molding cycles in the event that the work indexdoes not lie within the specified allowable range therefor. Preferably,increment size adjustment 66 specifies a percentage change which is tobe made.

The process control apparatus 50 may also include a selectively operabletime delay circuit having a timer with a variable adjustment 67. Thetime delay circuit may be used to postpone incremental changes in aviscosity affecting parameter when desired.

In operation, the injection molding cycle may be considered in threeportions: a plasticizing portion, an injection portion and a holdingportion. During the plasticizing portion, particulate material (seeFIG. 1) is introduced through the hopper 12 to the screw 16. The screwcompacts, compresses, heats and plasticizes the particulate, syntheticresinous material while it is conveyed forwardly along the length of thescrew 16. The band heaters 20, 22, 24 around the barrel 18 may transferadditional heat to the plasticized material and facilitate theplasticization thereof.

As the plasticized material accumulates at the left end of the screw 16as seen from FIG. 1, the plasticized material develops a fluid pressureacting upon the end of the screw, which tends to translate orreciprocate the plasticizing screw 16 to the right. To resist thetendency of the plasticizing screw 16 to translate in response to theaccumulating plasticizing material, hydraulic back pressure is exertedon the piston end 34 from the chamber 38. By controlling the backpressure in the chamber 38, the pressure in the plasticized materialaccumulated in the screw and the barrel 18 may be controlled.

The back pressure acting on the screw 16 is conventionally known to havea significant effect on the amount of work required during injection,and on the viscosity of the plasticized material accumulated between thescrew 16 and the barrel 18. Accordingly, the back pressure exerted onthe plasticizing screw 16 is a viscosity affecting parameter of theplasticizing apparatus 10 which has a potent effect on the viscosity ofthe material plasticized therein.

When a sufficient volume of plasticized material, or "shot", has beenaccumulated in the plasticizing apparatus 10, the injection portion ofthe molding cycle may commence. A high volume high pressure flow ofhydraulic fluid from pump 40 is introduced into the chamber 38. Theplasticizing screw 16 is then translated to the left (see FIG. 1) withrespect to the barrel 18 thereby discharging or expressing the shot ofplasticized material which had been accumulating therein. The shot isthus injected into the mold cavity defined between the stationary moldportion 28 and the movable mold portion 30 to form the molded article.

The transition between the plasticizing portion of the cycle and theinjection portion of the cycle may be indicated by using a preselectedpositon of the integration-initiating position transducer 54. Thepreselected position may be adjustable such that material in gates ofthe mold is cleared before the preselected position is reached by theposition transducer 54.

Typically, the injection portion continues until the mold cavity isfilled with plasticized material and until the plasticized material inthe mold attains a specified pressure. With the present invention, theend of the injection period is signaled when the mold cavity sensor 48attains the predetermined value input at 60.

While the material in the mold is solidifying, a suitable holdingpressure must be maintained, to accommodate for material shrinkage inthe mold which frequently occurs, if uniform, high quality articles areto be molded consistently. Accordingly, the third portion of the moldingcycle, the holding portion begins when the mold pressure has attainedthe selected value. During the holding portion, a low volume lowpressure flow of hydraulic fluid is maintained in the chamber 38. Thechange in flow volume and pressure between the injection portion and theholding portion may be triggered by the mold cavity pressure sensor 48.

The holding portion of the molding cycle continues until the plasticizedmaterial injected into the mold has solidified sufficiently for anarticle to be removed from the mold. Thereupon, the plasticizing portionof the cycle is initiated and continues as discussed above. During theplasticizing portion, the movable mold portion 30 may be withdrawn fromassociation with the stationary mold portion 28 such that the articlecan be removed therefrom. The mold cavity portions 28, 30 then return totheir abutting relationships such that the mold cavity is closed and isready to receive the next shot of plasticized material from theplasticizing apparatus 10.

Turning now to FIG. 2, the operation of the process control apparatus 50may be more readily visualized. The pressure sensing means 52 senseshydraulic injection pressure in the chamber 38 and provides apressure-related input signal to an integrator 68 which may comprise anoperational amplifier 67 having a capacitive feedback network 69. Theintegrator 68 integrates the injection pressure signal with respect totime and preferably begins at the inception of the injection portion ofthe cycle.

An initiator 70, which operates in response to theintegration-initiating position transducer 54, may be used to signal theintegrator 68 to begin. Typically the initiator may comprise an enablingcircuit 71 such as a switch connected in parallel with the integrator68. As the plasticizing screw 16 of FIG. 1 begins an injection stroke inresponse to an increased injection pressure in the chamber 38, theposition transducer 54 will encounter the preselected position and thensignal the integrator 68 through the initiator 70 (FIG. 2) to commencethe integration of injection pressure with respect to time.

The integration of injection pressure preferably continues throughoutthe injection portion of the molding cycle until a stop signal iscommunicated to the integrator 68 from a comparator 72. The comparator72 compares the pressure set by input 60 with a signal communicated toan analog comparator circuit 73 by the mold cavity material pressuresensing means 48.

When the mold cavity pressure attains the predetermined value, anenabling circuit 74 is inhibited by a binary signal from the comparatorcircuit 73 to stop the integration process. Simultaneously, thecomparator circuit 73 signals the valve 46 to shift to its low pressurelow volume flow position.

The output 76 of the integrator 68 constitutes an index of workperformed on the plasticized material in the shot during the injectionthereof into the mold and until the mold is filled with plasticizedmaterial to a predetermined pressure. More specifically, the timeintegral of hydraulic pressure acting on the piston end 34 of theplasticizing screw 16 (see FIG. 1) during the time a shot of material isinjected into the mold is a measure of the work required to overcome theflow resistance of the plasticized material and to inject the shot intothe mold. It is noted that viscosity is a measure of the resistance of afluid to internal flow and the work required to inject the shot into themold therefore may represent the apparent viscosity of the materialinjected. By integrating the work required to inject a shot into themold throughout the entire injection portion of the molding cycle, anyperturbations affecting the injection stroke or the viscous fluidflowing into the mold cavity are accounted for. Moreover, by continuingthe integration until a specified pressure is developed in the materialinjected into the mold cavity, a full mold with uniformly compactedmaterial is ensured. As the most influential parameters on productquality have been found to be material pressure and the work required tofill the mold with material, the combination of the calculated workindex and specified pressure enables the process control apparatus toconsistently and effectively produce high quality molded articles. Itshould also be apparent that making calculations during fractionalperiods of the injection portion of the molding cycle may not provide anindex as reliable as the index used here.

Having calculated a work index for the injection portion of the moldingcycle, the index is evaluated by comparator logic 78 (see FIG. 2) todetermine whether or not it lies in the predetermined allowable rangewhich is input to the process control apparatus 50 through the limits62, 63. If the work index lies within the specified range, the injectionmolding apparatus is operating within allowable constraints and nocontrol adjustments are required. On the other hand, if the work indexlies outside of the allowable range, a command signal is generated bythe comparator 78 to change a viscosity-affecting parameter throughsuitable feedback means.

The comparator logic 78 includes a suitable conventional analog todigital converter 80 which preferably converts the integrator output 76into a binary coded decimal (BCD) signal. The BCD signal is thencompared with the input high and low limits 62, 63 in a suitableconventional digital comparator 82 having a high output signal 81 and alow output signal 83. The digital comparator 82 generates a high orbinary one signal in the high output 81 if the calculated work index 76is greater than the high limit 62. Similarly, the digital comparator 82generates a high or binary one signal in the low output 83 if thecalculated work index exceeds the low limit 63.

A series of three suitable gates and inverting circuits are provided inthe comparator logic 78 to determine whether the work index 76 is above,within, or below the predetermined allowable range set by the inputlimits 62, 63. For example, a first AND gate 84 passes a command signalwhen both output signals 81, 83 are binary one and thus indicates thatthe index is above the allowable range. A second AND gate 86 passes asignal when the work index 76 exceeds the low limit 63 but does notexceed the high limit 62 and thus indicates the index is within thepredetermined allowable range. The third AND gate 88 passes a commandsignal when both output signals 81, 83 are binary zero and thereforeindicates that the calculated work index is below the allowable range.

The command signals from the first and third AND gates 84, 88 areelectrically communicated to a pair of enabling circuits 90, 92 whichalso receive a signal from the increment size adjustment 66. Theenabling circuit 90 passes a command expected to increase aviscosity-affecting parameter of the plasticizing apparatus in responseto the command signal from the third AND gate 88. The enabling circuit92 passes a command signal expected to decrease a viscosity-affectingparameter in response to the command signal from the first AND gate 84.For example, the enabling circuits 90, 92 may each comprise a suitableamplifier inhibited by a high binary signal level and enabled by a lowbinary signal level. The enabling circuit 90 may pass the signal fromthe increment size adjustment when enabled while the enabling circuit 92may pass an inverted or negative form of this signal when enabled.

The command signal from either enabling circuit 90, 92 is communicatedto one or more of the valves 42, 44, 46 through appropriate manuallypreset switches 94. As noted above, the valves 42, 44, 46 control theviscosity-affecting parameters of back pressure, injection pressure andscrew rotation speed, respectively.

The signal from each of the three AND gates 84, 86, 88 is supplied toanother comparator logic 96 which counts the number of consecutivemolding cycles in which viscosity corrections occur and compares thatcount to the allowable number of cycles input by the input 64.

The comparator logic 96 includes two OR gates 98, 100 which receivesignals from the AND gates 84, 86, 88. The first OR gate 98 is connectedto the first and third AND gates 84, 88 such that it will pass a signalwhenever a command signal emanates from the first comparator logic 78.The second OR gate 100 issues a shift signal for each molding cyclesince it operates regardless of the value of the work index asdetermined by the AND gates 84, 86, 88.

To provide pulses for a conventional, resettable COUNT circuit 104, aconventional HOLD circuit 102 is connected to both OR gates 98, 100. TheOR gate 98 passes a pulse which is retained by the HOLD circuit 102whenever the work index is not within the allowable range at the end ofeach cycle. The OR gate 100 passes a signal for each molding cycle whichinstructs the HOLD circuit 102 to shift the stored pulse out and shiftthe new pulse, if any, in from the OR gate 98. The HOLD circuit 102 maybe any suitable circuit for sampling and holding an input quantity inresponse to a shift signal. The COUNT circuit 104 may be any suitablebinary counter having a reset input terminal R.

In the event there is no pulse stored in the HOLD circuit 102, such aswhen the work index is within the allowable range, then the COUNTcircuit 104 is reset to zero through an inverter connected to an ANDgate 101 which, in turn, is connected to the reset terminal R. The ANDgate 101 also receives an input signal directly from the OR gate 100 sothat the COUNT circuit 104 is reset only when there is a shift signaland no pulse stored in the HOLD circuit. When the work index liesoutside the allowable range and pulses are stored during consecutivecycles, the COUNT circuit 104 accumulates the number of consecutivecycles.

A compare circuit 106 is connected to the COUNT circuit 104 andevaluates the number from the COUNT circuit 104 with respect to theallowable number of cycles input by thumbwheel 64. If the number ofcycles counted exceeds the allowable preset number, a signal is sent toa suitable indicator 108 which may comprise an audible alarm that warnsan operator the molding apparatus is out of control. Since the signalsfrom the COUNT circuit and the thumbwheel 64 may be binary in nature,the compare circuit 106 may be any suitable conventional circuit forcomparing two signals representing binary numbers.

The signals representing various input parameters, the computed workindex and other parameters of the process control apparatus 50 may besupplied to a suitable recorder 110 which may provide a printed record112 thereof for each molding cycle. For example, a conventional pen orchart type recorder or a recorder providing a digital record may beutilized for this purpose.

Yet another viscosity-affecting parameter which may be controlled by theprocess control apparatus 50 is the temperature of the barrel 18 (seeFIG. 1). For this purpose a plurality of temperature sensors 114 may beprovided at various locations along the barrel 18, the nozzle and oneach mold portion 28, 30. The output from each of the temperaturesensors 114 may be selectively scanned by a scanner 115 and thencompared with a preset temperature in comparator 116 which may receive asignal from the process control apparatus 50 as illustrated at 118. Thecomparator 116 may be selectively activated by a switch 120 of theprocess control apparatus 50 (see FIG. 2) and may also provide a printedrecord through a second suitable recorder 122 of the type previouslydescribed (see FIG. 1). When a temperature adjustment is necessary, thecomparator 116 may generate output signals to suitable solid statesilicon controlled rectifiers which control the band heaters 20, 22, 24,26 provided along the barrel 18. In this manner, the temperature of thebarrel 18 may be controlled in response to the calculated work indexgenerated in the process control apparatus 50.

Where temperature is selected as a viscosity-affecting parameter, it maybe desirable to include a suitable delay circuit to inhibit temperaturechanges until the work indexes calculated for each of severalconsecutive cycles are outside the predetermined allowable range therebyconfirming that an error trend exists. To enable the delay circuit, asuitable conventional switch 124 (see FIG. 2) interposed between theHOLD circuit 102 and OR gate 126 is moved to its second position. The ORgate 126 passes a signal to the COUNT circuit 104 when the switch 124 isin either its first or its second position.

With the delay circuit enabled, signals from the HOLD circuit 102 entera suitable conventional counter-decoder 128 having a reset terminal Rand an output for selecting either 3 or 4 as the number of moldingcycles which are to be delayed. The counter-decoder 128 counts pulsesfrom the HOLD circuit 102, decodes the current count, and passes abinary pulse from the output to the OR gate 126 when the selected numberof consecutive cycles has been attained.

The OR gate 126 passes a pulse to the COUNT circuit 104, as noted above,and to an input of AND gate 130. A second input of the AND gate 130receives a signal from the enabling circuits 90, 92. Accordingly, atemperature adjustment will be permitted by the AND gate 130 only whenthe selected number of cycles has occurred.

The counter-decoder output signal is also communicated to a time delaymeans which may comprise a suitable conventional timer circuit 132having the settable input 67. The counter-decoder output signal enablesthe timer circuit 132 which generates an output pulse to inhibit thecounter-decoder 128 for the period of time set by input 67. When the setperiod of time expires, the counter-decoder 128 is again operative tocount consecutive cycles during which the work index is outside theallowable range. Accordingly, the time delay means allows a reactiontime period before further temperature adjustments may be made.

The timer output pulse is also communiated to an input of an OR gate134. A second input of the OR gate 134 is connected to a suitableconventional two position switch 136 which operates as a slave with theswitch 124. The OR gate 134 is connected to the reset terminal R of thecounter-decoder 128 such that the counter-decoder 128 is reset wheneverthe OR gate 134 receives an appropriate reset pulse.

The first position of switch 136 is connected to a voltage source Vo.Accordingly, the counter-decoder will always be reset at zero when theswitch 124 is in its first position. When the switch 124 is moved to itssecond position to enable the delay circuit, the switch 136 is moved toits second position by virtue of the slave relationship. In the secondposition, the switch 136 receives any reset pulse emanating from the ANDgate 101 for the COUNT circuit 104. Since the switch 134 is connected tothe OR gate 134, the counter-decoder 128 is also reset along with theCOUNT circuit 104. In addition, the counter-decoder 128 is reset whenthe selected number of consecutive cycles is attained by virtue of theinput to the OR gate 134 from the timer circuit 132.

The process control apparatus 50, illustrated in FIG. 1, is shownseparated from the plasticizing apparatus 10 in the interest of clarity.In practice it is desirable to mount the process control apparatus 50 onthe plasticizing apparatus. In addition, the process control apparatus50 may generate binary-coded decimal output signals which are directlycompatible with computer storage and control.

Thus, it is apparent that there has been provided in accordance withthis invention, an injection molding process control which substantiallysatisfies the objects set forth above. Although the present inventionhas been described in conjunction with a specific embodiment thereof, itis evident that many alternatives, modifications, variations andequivalents will be apparent to those skilled in the art in light of theforegoing disclosure. Accordingly, it is expressly intended that allsuch alternatives, modifications, variations and equivalents which fallwithin the spirit and scope of this invention as defined in the appendedclaims be embraced thereby.

What is claimed is:
 1. In an automatically controlled injection moldingapparatus having a process control means for producing uniform, highquality articles from synthetic resinous material, the improvementcomprising:material pressure sensing means for sensing the pressure ofplasticized synthetic resinous material in the mold; injection pressuresensing means for sensing the pressure of hydraulic fluid during theinjection portion of the molding cycle; integration means operable tointegrate the work parameter of the injection molding apparatus duringthe injection portion of the molding cycle to obtain a work index;initiating means for starting the integration means approximately withthe inception of the injection portion of the molding cycle; terminatingmeans for stopping the integration means when the material pressuresensing means indicates a predetermined value; and feedback means formaking a compensatory adjustment in a viscosity-affecting parameter ofthe molding apparatus during subsequent molding cycles thereof and beingactivated by a signal from said process control means in response to thevalue of the work index.
 2. The apparatus of claim 1 wherein saidprocess control means includes means for indicating failure to restore awork index within a predetermined allowable range within a set number ofconsecutive molding cycles.
 3. The apparatus of claim 1 wherein thefeedback means makes a predetermined adjustment in the back pressureacting on a plasticizing screw during the plasticization portion ofsubsequent molding cycles.
 4. The apparatus of claim 1 wherein saidprocess control means includes means operable in response to theterminating means for shifting from the injection portion to the holdingportion of the molding cycle.
 5. Apparatus for cyclically controlling aninjection molding machine during injection molding of uniform, highquality articles from synthetic resinous material, comprising:firstsensing means for generating a first signal representing pressure ofsynthetic resinous material in a mold; second sensing means forgenerating a second signal representing a work parameter; adjustmentmeans for changing a viscosity-affecting parameter of subsequent cyclesin response to a command signal; and process control means forcontrolling a molding cycle having a plasticizing portion, an injectionportion and a holding portion, the process control means includinganintegrator for generating a work index by integrating said second signalwith respect to time during said injection portion until said firstsignal attains a predetermined value, a comparator for determining ifthe work index is within the predetermined allowable range, and meansfor generating an appropriate command signal to said adjustment meanswhen the work index is outside the predetermined allowable range,whereby the failure of the work index to lie within the predeterminedallowable range effects a variation in the plasticizing portion ofsubsequent cycles such that the work index of subsequent cycles willmore likely fall within the predetermined allowable range.
 6. Theapparatus of claim 5 wherein said process control means includes:cyclecounter means for counting the number of consecutive cycles in which thework index is outside the predetermined allowable range being operableto generate a call signal when the work index is outside thepredetermined allowable range for a predetermined number of consecutivecycles; and indicator means operable in response to the call signal forindicating failure of the work index to be within the allowable rangeafter the predetermined number of consecutive cycles.
 7. The apparatusof claim 6 wherein said first sensing means includes a pressuretransducer positioned at an extremity of the mold.
 8. The apparatus ofclaim 6 wherein said second sensing means senses injection pressureacting on a plasticizing screw.
 9. The apparatus of claim 8 wherein saidadjustment means effects an incremental change in back pressure actingon the plasticizing screw during the plasticizing portion of the moldingcycle.
 10. The apparatus of claim 9 wherein the process control meansincludes a settable predetermined allowable range for the work index, asettable predetermined value for the first signal, a settable number ofallowable consecutive cycles, and a settable incremental change for theviscosity-affecting parameter.
 11. The apparatus of claim 5 wherein saidprocess control means includes inhibiting means for delaying changes ina viscosity-affecting parameter for a selected number of cycles toeliminate changes induced by erratic command signals.
 12. The apparatusof claim 5 wherein said process control means includes a time delaymeans for allowing a reaction time period to expire before furtherchanges are made to the viscosity-affecting parameter.